This invention relates generally to rotating induction apparatus and more specifically to rotating induction apparatus that are tolerant of harmonics.
In a rotating induction apparatus, a current of electrical charge generated within a magnetic field experiences a force perpendicular to the flow of charge and to the lines of force of the magnetic field. If a conductor is forced through a magnetic field by some sort of an external prime mover, an electrical current is caused to flow; this is the principle of the operation of an electrical generator. When an electrical current flows through a conductor in a magnetic field, a force is applied to the conductor; this is the principle of the operation of an electrical motor.
In an alternating current (AC) induction motor, a rotating magnetic field is produced by the stator or stationary portion of the apparatus. This rotating magnetic field interacts with current carried by conductors of the rotor, causing the rotor to turn. It also produces currents in the rotor conductors by transformer action. Thus, the rotor needs no connections to an electrical supply, and is simply supported by bearings which allow free rotation.
The rotating magnetic field is produced by coils of wire or windings, suitably positioned on the stator. Each winding, when energized with a direct current, would produce a fixed magnetic field. By energizing a winding with a sinusoidal AC, a smooth varying magnetic field of fixed orientation may be produced. By positioning several windings of differing orientations on a single stator and energizing the windings with alternating currents of differing phase, a rotating magnetic field is produced which is the sum of the time-varying fixed orientation magnetic fields generated by each winding/phase. The stator is defined as the stationary part of the magnetic circuit and these stator windings.
The rotating field produced by the stator windings is complex and irregular. By the principal of superposition, the rotating field may be analyzed as being composed of numerous rotating fields of different shape, including a fundamental or desired lowest frequency structure. The rotating field is composed of this fundamental field and higher frequency harmonic fields.
The excitation currents may similarly be complex, and may be analyzed as being composed of several different harmonic currents. The fundamental excitation current is the primary source of torque.
Spatial harmonics, or air-gap harmonics, are harmonic fields generated by the non-sinusoidal nature of the field generated by each winding. When spatial harmonics are excited by the fundamental drive currents, they produce a secondary rotating field that rotates slower than the fundamental field. For a given excitation frequency, spatial harmonic fields rotate more slowly than the fundamental field.
Harmonic fields generated by non-sinusoidal drive wave-forms are termed temporal harmonics. Rotating fields produced by temporal harmonic currents rotate more rapidly than the fundamental field. When temporal harmonics excite the fundamental spatial field, they produce a secondary rotating field that rotates more rapidly than the fundamental field and may rotate in the opposite direction to the fundamental field.
Therefore, both spatial and temporal harmonics in rotating fields may adversely affect the efficiency of a conventional rotating induction apparatus, lowering torque and increasing current flow.